1 Technical Field
This disclosure relates generally to an electric power steering system (also called a steering assist system) which is equipped with a motor controller engineered to control operation of an electric motor to produce steering assist torque and/or automatic steering torque.
2 Background Art
Japanese Patent First Publication No. H09-221053 teaches an automotive steering system which analyzes image information derived by a camera which captures an image of a view ahead of a vehicle to calculate a positional relation between a system vehicle equipped with this system and a traffic lane on which the system vehicle is now running, and performs a lane-keep control mode and a power steering control mode using a single actuator (e.g., an electric motor). The lane-keep control mode is to keep the system vehicle in the lane. The power steering control mode is to produce assist torque to assist a vehicle driver in assisting steering the system vehicle.
The above type of steering system is usually equipped with a torsion bar and a torque sensor. The torsion bar is arranged closer to a steering wheel than an electric motor is. The torque sensor works to measure twisting of the torsion bar to determine the degree of torque for steering the system vehicle (which will also be referred to as steering torque below). The torsion bar, however, also experiences twisting when the motor is actuated to output an automatic steering torque as well as when the steering wheel is turned. The steering system, therefore, encounters the drawback in that the twisting of the torsion bar arising from the actuation of the motor to produce the automatic steering torque will result in generation of mechanical vibration in the steering wheel (correctly, a portion of a steering power transmission mechanism which lies between the steering wheel and the torsion bar.